Khalid Ahmad Khattak

Lesson 3 – Variables

A variable in Java is a container that stores a value. You can store numbers, text, decimals, and many other types of data. Variables allow your program to remember information.

1. Declaring a Variable

To create a variable, you must write:

dataType variableName = value;
        

Example 1 – Full Program

public class Main {
    public static void main(String[] args) {

        int age = 20;
        System.out.println("My age is " + age);
    }
}
        

2. Different Types of Variables

• int – whole numbers
• double – decimal numbers
• String – text
• boolean – true/false

Example 2 – Full Program (Multiple Variables)

public class Main {
    public static void main(String[] args) {

        int year = 2025;
        double temperature = 36.5;
        String name = "Khalid";
        boolean isStudent = false;

        System.out.println(year);
        System.out.println(temperature);
        System.out.println(name);
        System.out.println(isStudent);
    }
}
        

3. Changing the Value of a Variable

Example 3 – Full Program

public class Main {
    public static void main(String[] args) {

        int score = 50;
        System.out.println("Old score: " + score);

        score = 80; // value updated
        System.out.println("New score: " + score);
    }
}
        

4. Adding Variables Together

Example 4 – Full Program

public class Main {
    public static void main(String[] args) {

        int x = 5;
        int y = 7;
        int total = x + y;

        System.out.println("Total = " + total);
    }
}
        

5. Naming Rules for Variables

• Must start with a letter or underscore
• Cannot start with a number
• No spaces allowed
• Case-sensitive
• Use meaningful names

Example 5 – Correct & Wrong Names

public class Main {
    public static void main(String[] args) {

        int age = 30;     // correct
        int Age = 40;     // correct but different
        int _value = 100; // correct

        // int 2number = 50;    WRONG
        // int student name = 20; WRONG

        System.out.println(age);
        System.out.println(Age);
        System.out.println(_value);
    }
}
        

6. Concatenation: Joining Text and Variables

Example 6 – Full Program

public class Main {
    public static void main(String[] args) {

        String firstName = "Khalid";
        String lastName = "Khattak";

        String fullName = firstName + " " + lastName;

        System.out.println("Full name: " + fullName);
    }
}
        

7. Final Variables (Constant Values)

If you want a variable that cannot change later, use final.

Example 7 – Full Program

public class Main {
    public static void main(String[] args) {

        final double PI = 3.14159;
        System.out.println("Value of PI: " + PI);

        // PI = 5;  // ERROR: cannot change final variable
    }
}
        

8. Declaring Multiple Variables

Example 8 – Full Program

public class Main {
    public static void main(String[] args) {

        int a = 10, b = 20, c = 30;

        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
    }
}
        

9. Using Variables Inside Messages

Example 9 – Full Program

public class Main {
    public static void main(String[] args) {

        int marks = 90;

        System.out.println("You scored: " + marks + " marks.");
    }
}
        

10. Summary

• Variables store data
• Different data types store different kinds of values
• Variables can change except when marked as final
• Meaningful names improve your code

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